Hybrid vehicle architecture may take several forms for operatively connecting a battery, an electric traction motor and a combustion engine together in the driveline of the vehicle. One proposed architecture in development by the assignee of this application is a Modular Hybrid Transmission (MHT). One embodiment of the MHT is the Electric Converter-Less Transmission (ECLT). To replicate the torque converter function of a conventional automatic transmission, the MHT powertrain without a torque converter may rely upon active controls of a starter/alternator and a disconnect clutch between the combustion engine and the electric motor. Alternatively, the MHT may also be provided with a torque converter. A launch clutch or torque converter may be provided between the electric motor and the wheels.
The drivability of the MHT must be comparable to drivelines having a production automatic transmission. A major control challenge of the MHT is to absorb “clunks,” pulsations and vibrations in the driveline during engine start and clutch engagement, creating a quieter, less stressful driving experience.
New challenges arise with MHT systems as to the coordination of the clutch, engine and motor, especially during the complicated clutch engagement transients. All the friction element control, pressure control, and the motor toque control should be integrated seamlessly to deliver smooth wheel torque. In addition, converter-less disconnect clutch engagement is very sensitive to the clutch pressure and it is a challenge to achieve the proper damping and smoothness during the clutch engagement.
The engine in a MHT must start smoothly and quickly. Every start is accompanied by a transient clutch engagement phase during which time substantial inertial drag and torque disturbances are transferred to the driveline. The difficulty and uncertainty of estimating the engine and clutch torque caused by complicated transient dynamics are a challenging task for motor torque compensation.
During the MHT clutch engagement transient for engine starts, there are problems of oscillations arising from the excitation of the mechanical resonance by various disturbances. This resultant oscillation phenomenon is due to low damping in the driveline due to the absence of a torque converter. Applying the electric motor torque generates torque ripples with frequencies that are motor speed dependent.
The above problems and other problems are addressed by the present disclosure as summarized below.